1. Field of the Invention
The present invention relates to a rubber reinforcing steel cord, manufacturing method thereof and to a pneumatic tire, in which a cobalt compound is contained in oil to be applied to a surface of a steel wire plated with metal containing copper, to prevent degradation of rubber while maintaining adhesion with rubber and to improve durability of the tire while reducing cost.
2. Description of the Related Art
A steel cord used for reinforcing a rubber product such as tire must have good adhesion with rubber, in order to form a composite with rubber. Various techniques have been disclosed to attain sufficient adhesion in steel cord and rubber.
For example, Japanese Patent Laying-Open No. 6-49786 discloses a technique in which a steel wire forming the steel cord is coated with a lubricating oil, and a water absorbing polymer is introduced in the gap between each of the steel wires.
Japanese Patent Laying-Open No. 1-153783 discloses a technique in which a reaction mixture mainly containing a cobalt salt of organic carboxylic acid•boron metallic soap is applied to the steel cord.
Japanese Patent Laying-Open No. 4-11637 discloses a technique in which a cobalt metal salt, sulfur and a bismaleimide compound are contained in the rubber composition, and the surface of the steel cord is provided with a cobalt plating layer.
By the above described techniques, however, it has been difficult to ensure good adhesion between rubber and steel cord, and to provide an economical tire using the steel cord.
Therefore, a technique has been proposed in which the surface of the steel cord is brass-plated, while a cobalt metal salt or the like as an adhesion assisting agent is added to a rubber composition, and the two are adhered with each other.
The steel cord with brass plating, however, has problems such as rust gathered on the surface of the steel cord during transportation or during the steps of manufacturing tires, and degradation of adhesion with rubber resulting from oxidation and alteration of the plated surface.
Addition of a cobalt compound to rubber is disadvantageous in that molecular chain of rubber may possibly degrade in the unvulcanized state, and that the cost of addition is extremely high.